The SV40 Enhancer Can Be Dissected Into Multiple Segments, Each with a Different Cell Type Specificity

Downloaded from genesdev.cshlp.org on September 29, 2021 - Published by Cold Spring Harbor Laboratory Press The SV40 enhancer can be dissected into multiple segments, each with a different cell type specificity Sabine Schirm, ~ Josef Jiricny, 2 and Walter Schaffner, 1 1Institute fiir Molekularbiologie II der Universit/it Ziirich, H6nggerberg, CH-8093 Ziirich, Switzerland; 2Friedrich Miescher Institut, PO Box 2543, CH-4002 Basel, Switzerland The SV40 enhancer is known to be active in a wide variety of tissues and species. It contains a number of sequence motifs that can be bound by protein factors and whose integrity is essential for full enhancer activity. We have individually analyzed three synthetic oligonucleotides derived from sequences present within the SV40 enhancer: two oligonucleotides contain variants of the enhancer "core" sequence (designated corePVUII and coreC) and the third represents a region containing a decanucleotide homology to the immunoglobulin promoters/enhancers {designated SPHI). The oligonucleotides were multimerized and linked to a [3-globin test gene. Transcripts of the test gene were analyzed following transient expression in 10 cell lines representing a broad spectrum of tissues. We show that each of the three short segments can individually act as an enhancer when present in multiple copies. None of these enhancers is ubiquitously active; however, each shows activity in a distinctive subpopulation of cell lines. This cell type specificity is most remarkable in the case of the two oligonucleotide segments containing the core sequences. One of these is primarily active in CV-I cells, whereas the other exhibits a cell type specificity identical to that of the entire enhancer, possibly identifying it as the most important sequence element within the native SV40 enhancer. Our data suggest that a particular cell type specificity is typical for individual enhancer segments, and that enhancers of differing specificity can be assembled from the individual sequence motifs by combining them in different patterns. [Key Words: SV40 enhancer; remote transcription control; cell type-specific transcription; synthetic oligonucleotides] Received December 5, 1986; revised version received and accepted January 8, 1987. The phenomenon of remote transcriptional control by transcription when introduced into frog kidney (R. cis-acting DNA elements was originally discovered in Miller and W. Schaffner, unpubl.)or even algal cells mammalian cells and described as the enhancer effect (Neuhaus et al. 1984). The sequences important for SV40 (Banerji et al. 1981; Moreau et al. 1981). The first en- enhancer activity have been investigated by studying hancing DNA sequence identified was that from the deletion-duplication events (de Villiers et al. 1984; simian virus 40 (SV40) genome. When linked to homolo- Swimmer and Shenk 1984; Weber et al. 1984; Herr and gous and heterologous promoters, it was found to stimu- Gluzman 1985; Herr and Clarke 1986)and, in a more late transcription in an orientation-independent refined way, by site-directed mutagenesis (Zenke et al. manner, even from a position downstream of the gene 1986; see also Weiher et al. 1983). These studies sup- (Banerji et al. 1981). Following this discovery, a great ported our suggestion that enhancers generally represent number of viral and cellular enhancers have been re- a modular arrangement of short sequence motifs (Bos- ported. However, the SV40 enhancer is the best charac- hart et al. 1985; Dorsch-H/isler et al. 1985; Serfling et al. terized and the most often used example of a prototype 1985), each interacting with a specific cellular transcrip- enhancer. In contrast to enhancers which exhibit a strict tion factor. The first indication for the relevance of cel- cell type specificity (Banerji et al. 1983; Gillies et al. lular trans-acting factors to SV40 enhancer function 1983; for review, see Gluzman 1985; Serfling et al. 1985) came from in vivo competition studies (Sch61er and or, at least, a cell preference (de Villiers and Schaffner Gruss 1984). Cellular protein factors physically inter- 1981; Laimins et al. 1982), the SV40 enhancer is known acting in vitro with the SV40 enhancer were subse- to act in a wide variety of tissues and hosts. In addition quently detected by DNase I footprinting (Davidson et to its activity in mammalian cells, it also stimulates al. 1986; Wildeman et al. 1986). Additionally, Davidson GENES & DEVELOPMENT 1:65-74 (1987) © by Cold Spring Harbor Laboratory ISSN 0890-9369/87 $1.00 65 Downloaded from genesdev.cshlp.org on September 29, 2021 - Published by Cold Spring Harbor Laboratory Press Schirm et al. et al. (1986)provided evidence for a cell type-specific order to analyze their enhancing activity individually binding of certain protein factors to overlapping sets of and to investigate their cell type specificity (Fig. 1A). For sequence motifs within the SV40 enhancer. Evidence for the choice of these segments, we took into consideration the binding of more than one factor to a given enhancer the reports by Herr and colleagues (1985, 1986)and espe- was also indirectly shown by Sen and Baltimore (1986), cially that of Zenke et al. (1986), which showed that the who found that different segments of the immunoglob- integrity of each of these regions is essential for full en- ulin kappa enhancer did not compete with each other in hancer activity. Three segments, designated corePVUII, protein-binding studies. coreC, and SPHI, were synthesized as the double- Hints that a segment of an enhancer may have a cell stranded oligonucleotides shown in Figure lB. The type specificity different from the complete enhancer corePVUII-oligonucleotide was synthesized as a 22-bp first came from studies of SV40 enhancer variants, as dimer of an l l-bp sequence. The boundaries were well as from analyses of enhancers within the polyoma- chosen in such a way that its multimerization did not virus and Herpesvirus saimiri genomes (de Villiers and create a new junction but rather resulted in a circular Schaffner 1981; Schirm et al. 1985). An SV40 enhancer redundancy of sequences naturally occurring in the deletion variant that had duplicated a residual enhancer SV40 enhancer. Thus, the 22-bp dimer of this DNA seg- segment restored enough activity for growth in CV-1 ment contains a continuous 14-bp sequence from the ~ kidney epithelial cells (Weber et al. 1984). However, SV40 enhancer, and the stretch of alternating purines when tested in a wider variety of primate cells, this du- and pyrimidines {bp 258-265)is conserved (Fig. 1C, part plicated enhancer variant was active only in CV-1 and in a). Both the coreC- and the corePVUII-oligonucleotides lymphoid cells (M. Pettersson and W. Schaffner, un- include the "core" consensus sequence (Weiher et al. publ.). An altered cell type-specificity of a mutant SV40 1983). The monomer of the corePVUII-oligonucleotide enhancer was also quoted in Herr and Clarke (1986). In contains no other sequences besides this homologous addition, a small segment of the enhancer of Herpes- stretch (Fig. 1C, part b). In an initial experiment these virus saimiri has a pronounced cell type specificity dis- oligonucleotides were ligated to generate multiple tinct from that of the complete enhancer (S. Schirm and copies, and then inserted into the Sinai site of the vector W. Schaffner, unpubl.). A weak but significant cell type pSVXB (Tognoni et al. 1985), upstream of the SV40 early preference had also been observed for each half of the promoter and T-antigen gene. Plasmid was prepared polyomavirus enhancer when tested separately (Her- from pools of bacterial clones (one pool for each of the bomel et al. 1984; see also Amati 1985). three oligonucleotides)and transfected into different cell For our analysis of small segments of the SV40 en- lines. Enhancer activity of the polymerized synthetic se- hancer, we have exploited the observation that a high quences was monitored by staining for T-antigen with activity can result from multiple tandem copies of a indirect immunofluorescence. In this assay the coreC- subfunctional enhancer segment (de Villiers et al. 1984; oligonucleotide was found to be active in all cell lines Laimins et al. 1984; Weber et al. 1984) or even a short tested, namely CV-1 {monkey kidney), HeLa {human oligonucleotide (Veldman et al. 1985; our own data; B. cervix carcinoma), Molt-4 (human T cells), and 3T6 Ondek and W. Herr, pets. comm.). We report that short (mouse fibroblasts), while the activity of corePVUII sequences dissected from the SV40 enhancer each have a and SPHI were restricted to CV-1 and lymphoid cells, re- characteristic cell type specificity, which can be distinct spectively (data not shown). To follow up these prelim- from that of the complete enhancer. Furthermore, we inary observations in more detail, defined multimers show that these specificities can be different even be- (corePVUII, eight copies; coreC, four and eight copies; tween two segments containing motifs of high sequence SPHI, eight copies)were inserted 1.2 kb downstream of a homology. Our findings indicate that enhancers are rabbit f~-globin test gene in plasmid JK- (Fig:. 2A). composed of multiple sequence elements which, in prin- A total of 10 different cell lines were transfected with ciple, can substitute for each other, but in a cell type- these constructs, using either the calcium phosphate specific manner. It also appears that nucleotides next to method (Graham and van der Eb 1973; Wigler et al. a characteristic enhancer motif can modulate the cell 1978) or the DEAE-dextran method (Luthman and Mag- type specificity of the motif. In addition, the finding of nusson 1983), each with modifications according to the cell type-specific activity of multimerized segments de- requirements of the cell line (see Materials and rived from an enhancer that as a whole is active in most methods; for the origins of cell lines see Table 1). In all cells suggests a complex interplay of enhancer sequence transfection experiments a negative control and a posi- motifs and their cognate factors.

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